A keyboard is one of many input devices included in a computer configuration. A user uses fingers to depress keys on the keyboard to input data.
FIG. 1 is a schematic perspective view of a keyboard for a notebook computer, and FIG. 2 is a fragmentary and enlarged perspective view of the keyboard of FIG. 1 showing a structure of one single key thereof. As shown, the keyboard mainly includes a base on which a plurality of keys are provided. More specifically, the keyboard includes a sheet metal base 1, a membrane-type switch circuit board 2 located above the sheet metal base 1, a plurality of balancing mechanisms 3, a plurality of  key caps 4, and a plurality of elastic cups 5.
The sheet metal base 1 serves as a mounting platform in the keyboard. There are a plurality of upward extended lower shaft holders 11 and lower slide ways 12 formed on the sheet metal base 1 for holding the balancing mechanisms 3 thereto. The base 1 may function equally well to hold the balancing mechanisms 3 when it is only provided with the lower shaft holders 11 or the lower slide ways 12.
The membrane-type switch circuit board 2 is located above the base 1 and includes a plurality of membrane-type switches 21 and connecting circuits 22.
Each of the balancing mechanisms 3 is an X-type balancing mechanism being provided at four outmost corners or other suitable positions with lower shafts 31 and/or lower lugs 32 for engaging with the lower shaft holders 11 and/or the lower slide ways 12, respectively, and upper shafts 33 and/or upper lugs 34 for engaging with upper shaft holders 41 and/or upper slide ways 42, respectively, correspondingly provided on the key cap 4. The balancing mechanism 3 keeps the corresponding key cap 4 in a balanced position when the key cap 4 is downward pushed, and normally locates the key cap 4 in place. 
Each of the key caps 4 is provided at a lower surface with upper shaft holders 41 and upper slide ways 42 for engaging with the upper shafts 33 and the upper lugs 34, respectively, on the balancing mechanism 3. The key caps 4 may still be firmly held to the balancing mechanisms 3 even if the key caps 4 are provided with only the upper shaft holders 41 or the upper slide ways 42.
As can be seen from FIGS. 3 and 4, each of the elastic cups 5 is located between the membrane-type switch circuit board 2 and one of the key caps 4 to align with a center of a corresponding membrane-type switch 21. The elastic cup 5 has an elasticity and a cup-shaped configuration that provide a sufficient restoring force for the downward pushed key cap 4 to return to its original position before being depressed. The elastic cup 5 is provided at an inner top with a central stem 51, as can be seen from FIGS. 6 and 7. When a key cap 4 is depressed, the central stem 51 of the corresponding elastic cup 5 is moved downward to press against the corresponding membrane switch 21 to make it.
Please refer to FIG. 5 that is a perspective view of a conventional elastic-cup support 6 in the keyboard for locating the elastic cups 5 in place. The elastic-cup  support 6 is a flat sheet having a plurality of locating openings 61 formed thereon at predetermined positions. The elastic cups 5 are separately mounted on the flat sheet of the elastic-cup support 6 in a non-detachable manner, so that the elastic cups 5 and the support 6 together form an elastic-cup assembly A. When the elastic-cup assembly A is sandwiched between the key caps 4 and the membrane-type switch circuit board 2, the locating openings 61 are used to locate the elastic-cup assembly A in place on the circuit board 2, so that each elastic cup 5 is aligned with a corresponding membrane-type switch 21.
The elastic-cup assembly A may be divided into two types according to the manner in which the elastic cups 5 are connected to the flat sheet of the elastic-cup support 6. In FIG. 6, the flat sheet 6 and the elastic cups 5 are made of the same type of rubber material through integral molding to form the elastic-cup assembly A. In FIG. 7, the flat sheet 6 and the elastic cups 5 are made of different types of materials and are separately produced. For example, the flat sheet 6 is formed at positions corresponding to the membrane-type switches 21 on the circuit board 2 with through holes 62. The elastic cups 5 are separately connected at a lower edge 52 to a circumferential edge of the through holes 62 by  means of bonding agent 63 to form the elastic-cup assembly A. In either case, the elastic cups 5 are always an integral part of the flat sheet 6 and are not detachable therefrom.
In the course of manufacturing the elastic-cup assembly A, or in the course of using the keyboard, it is possible a certain one of the elastic cups 5 is improperly produced or damaged and requires replacement. In the conventional elastic-cup assembly A, either that shown in FIG. 6 or FIG. 7, since the elastic cups 5 are not detachable from the flat sheet of the elastic-cup support 6, any damaged or defective elastic cup 5 will make the whole elastic-cup assembly A unusable because it is impossible to remove only the damaged or defective elastic cup 5 from the elastic-cup support 6 and replace it with a good one. That is, the whole elastic-cup assembly A having a defective or damaged elastic cup 5 must be replaced with a new one. This would inevitably increase the bad yield in manufacturing the keyboard and form unnecessary waste of materials to increase the manufacturing cost.